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Chapter XIII. The Germinal Area. The Embryo And Its Appendages

I. The Germinal Area

Definition. — The germinal area {area (jenninaiiva^ area embryojialiSy Keivihofy aire germinative) is that portion of the meroblastic vertebrate ovum in the centre of which the embryo is diflferentiated. It therefore comprises both tlio embryo proper and the region immediately surrounding it. It exists in all amniota, but of course in the higher mammals, owing to the loss of yolk in the ovum, the primitive relations are less clear than in Sauropsida. The area is further characterized by various gradually develope<l peculiarities, three of which deserve si)ecial mention. To take them in the order of their appearance, the three peculiarities are,^/\sf, the extension of the archenteric cavity under nearly the whole of the area ; second, the extension of the coelom over nearly the whole of the area ; th ird, the development of blood-vessels and bhxxl beginning peripherally in the splanchnic leaf of the mesoderm and extending jp'adually into the embryo.

1. Extension of the Archenteric Cavity - As shown in the previous chapter, only a small part of the archenteron of amniota is taken up into the embryo, and the rest of the cavity remains as the cavity of the yolk-sfic, and therefore the entoderm of the area belongs, for the most part, to the future yolk-sac. As jwinted out in the section on the entodermal cells in the preceding chapter, it is only on the upper side of the expanded archenteron that the entoderm becomes distinctly cellular; on the lower side the yolk is multinucleate, but not divided into discrete cells ; at the edge of the expanded cavity the upi>er cellular layer passes gradually into the yolk and the region of the transition is known as the germinal wall, the structure of which is discussed in the chapter on the yolk-siic. As previously pointed out, the cells very early assume two forms, becoming thin and flattened in the central region of the area, and remaining as long cylinder cells in the i>eripheral zone ; this difference results in a greater transparency in the central zone, which has accordingly received the name of area pellucida, while the peripheral zone, owing to its relatively great opacity, has l>een named the area opaca. Another result of the extension of the archenteron is that all the layers above it can 1)0 easily removed from the rest of the o\^im, keeping their natural connections otherwise intact ; they form when thus remfeved a thin membrane, which, following the terminology of the older embr>'ologists, we commonly speak of as the blastoderm ; compare the section on the meroblastic embryo, p. 128.

2. The extension of the coelom of course divides the mesoderm into an upj)er (somatic) and lower (splanchnic) layer. But the division does not take place in certain definite regions, which are, 1, the primitive streak; 2, the nxia of the embryo; 3, the proamniotic area, in which for a long period there is no mwiodenii in Hmniota. It might also be added that as the mesoderm is excluded from the oral and anal membranes there is no coelom in them. Throughout the rest of the germinal area, the co-lom gradually extends, but for a long time it fails to reach, and in certain animals never reaches, the peripherj' of the constantly expanding mesoderm. The history of the embryonic coslom in given in special chapters, the history of the extra-embryonic coelom is indicated in the section of this chapter upon the origin of the amnion.

3. The appearance of the blood-ressels onil blood has been considered in Chaper X. ; it leada to the differentiation of the area vaseulosa {Qe/dssliof, tiire t'osciilaire), which is the region of the extra-embr)'onic circulation. As soon as the embrj-onic area contains a distinct vascular network, there appears a jjeripheral vessel which marks the boundary of the area vaaculosa, and is called the sinus terminalis. Thovasculosa does not rea<^h to the outer boundary of the germinal area, so that the region of the blood-vessels is inclosed in a ring which i'^ known as the area n'tellina.

Topt^raphy — The first differentation in the germinal area, wh en can be clearl rerogn ze<l bj tl e naked eye, is the appearance if the area pelluciua, which

3 shortly followed by that of the primitive streak. Fig, 78, p. 131. Further progi-ess results in the gi-adual ' Hfferentiation of the embryo, the steady expansion of the germinal area over the yolk, II the sharper demarcation if the area jKilhicida, which tiocomes pear-shapeii, and in the appearance of the bloodveaself- Fig. 155 represents heembryonicareaof a ben's ivum after about thirty hours' incubation. The embrj-o is well advanced in de[ velopment, for although the primitive streak, pr, still reiiains in part and the medullary gnvove, J/rf, ia still oiien Iiehind, the brain is already a mark(^<l out and thf head has

iwrtly free; alongpr «n sidetbcmeduilHlionine pairs

Ha ^£ Hp^yignts {pnito-vevtebrjB,

auct ) around tl e eml rj o one en 1 recognizes the pear-shaiK'd area ])elluc da 4/ anltl larkera p ra, -4o, by which it is inclosed; the rea c lo i^tands o t «] cuously and is boimded by the already 1 t ngu si able s nus t rm nalis, .if; around and underneath the le 1 s tie transl -ent i r>anniotic urea, pro. (tin, from which the mesoderm is Jiltuyethcr absent, and which therefore cannot contain iiny blood- ve»selij, nor are there at this stage any vessels in fi-ont of tho proamnion.

In tho ovum of the mammalia there occurs a modification of the ocbiderm, where that layer is attached to the ^^-alls of the maternal uterus, Tho region over wliich the attachment takes place fjives rise in the higlier mammals to the rluceuta. Hence the area of modified ectoderm may bo called the phicental area. It lias bet-n, aa yet, verj- little studied. As it is not possible at i)res('ut to S[>eak in genend tennsof the embryonic area of mammals, I cnufine myself t» a description of the area in the much-studied rabbit, following

Van Benoden and Julia, 84.1. Tho gcrminafive an-a, Fig. 15G, is nearly circular, iirid at the stage figured slu.u-s the foll()wing ixiiiliaritifs. The nearly straight embryo lies in the centre and exhibits ])Iaiiily the central nervous system and the i)ri)to-vertebrBe ; around the head of the embryo is a clear s]iace, i>r. »., the proamniotic an-a, ov<'r which no mosoilerm is develoi<e<l ; around the sides and hind end of the embryo is another light place which contains mesi h.Ut in. but is distingni'sheil by the retarded vascularization; this is the amniotic area, ti.ii.. and is converted by a pi-ocess of upfolding inio tJie amnion, which covers the ijosterior ]>ortion of the rabbit embryo. The remainder of the germinal disc constitutes the area vasculo.-ia, a. c, with the t<LTmimd sinus, blotnl- islands, etc. The ari'a ousists of two membram-s, the upiier, tho somatupleure, lie lower, the Bplanchnopletire ; a large portion of the former behind the embryo has been torn off, a. pi.; this defect is due to the fact that over this region villoBJties have appeared, and a close connection established between this region and the uterine wall ; it is by this means that the ovum is attached; hence, when the embryo is removed from the uterus, this area of the splanchnopleure (chorion) remains adherent to the uterus. As development proceeds, the allantois grows up against this area, over which the ditferentiation of the placenta takes place; hence the name, area placentalis.

The following description probably represents what was the primitive condition of vessels in the mnnimalian area vasculosa. It applies to an early stage in the rabbit, which has been figured by Bischoff, 42.1, Tab. XIV., Fig. (50, whose figure is copied in Kolliker's "Grundriss," Fig. tlO, p. lUtl. An essentially similar arrangement of the vessels exists also at a corresponding stage in the dog, Bischoff, 46.1, Taf. VII., Fig. 37, C. The veins are much more symmetrical than in the chick, and have the same general plan ; the sinus terminalis belongs to the venous system, so that the connection with the arterial circulation, found later, is secondary ; the aorta of the embryo is double, and gives off on each side (segmentally arranged?) transverse branches, one of which develops into the large trunk shown in Fig. 158; the network of small vessels forms two layers, of which the upper is connected with the arteries, the lower with the veins. The change from the earlier condition to the later has still to be followed.

Selenka has figured the vascular area of an opossmn, 86.1, Taf. XXIII., Fig. 3, in a condition which suggests at once a transition from between that just described and that described in the next paragraph; the figure shows the veins without direct connection with the sinus, while the aorta, though it gives off numerous small branches, has extended tailward of the embryo and joined the sinus.

According to Van Beneden's recent researches on the rabbit the arrangement of the main vessels in the area vasculosa at a later stage is quite different. The sinus terminalis forms a complete ring, Fig. 158, and is connected with the arterial system by a single trunk, which corresponds to the left omphalic arter}' of the bird. For some time the connection between the embryonic arteries and the area vasculosa is entirely through capillaries, and the arterial trunk on the vascular area does not api)ear in the rabbit for several days. There are two veins, one arising from each side of the body and passing out on to ' the area vasculosa over the back of the embrj^o; they are the two large upper vessels in the figure.

Growth of the Vascular Area

As the blood-vessels appear at first only in the splanchnic mesoderm, the vascular area belongs to the splanchnopleure, or, in other words, is part of the wall of the yolk-sac; hence the circulation of the area is often spoken of as the vitelline circulation. The growth of the vascular area is therefore part of the history of the yolk-sac, and is considered now from convenience merely. The expansion of the vascular area is due to the growth and differentiation of the mesoderm, and in those mammals in which, as in the rabbit, the mesoderm extends only part way over the yolk, the vascular area cannot spread over the whole blastodermic vesicle; but in those mammals in which, as in man, the mesoderm grows completely around the yolk, the vascular area may also extend completely around the yolk, with the consequence of the disappearance of the sinus terminalis. In the earliest known stages of man, the yolk-sac was found completely vascularized.

The gradual spread of the area vasculosa over the yolk may be readily followed in the hen's egg. It is due, as just stated, to the growth and differentiation of the mesoderm. The size of the vascular area is very variable, but the following table represents the approximate sizes, for several ages, as measured on blastoderms remov€Hl from the yolk, fiatteiied and hardened; the total circumference of the hen's yolk is al)out iiO mm. The area vasculosa of the chick measures —

At 2 days about 9 mm. in transverse diameter.

It is not until the seventeenth day of incubation that the yolk is completely overgrown by the vascular area, Duval '* Atlas," Fig. 651.

II. The Form of the Embryo.

It has been ix)inted out already that among vertebrates there are two i)rincipal types of embryonic fomi : one, which is the more primitive, characterized by the yolk-mass l)eing included in the Ixxly of the embryo; the otlior is secondary and character ize<l by the separation of the embryo and the yolk.

The primitive type of vertebrate embryo is fcnmd in the lampreys, ganoids, and amphibians; the ventral side of the embryo is very much distended to allow room for the yolk, which consists, after the segmentation is completed, of a mass of cells, which lie for the most part below the archenteric cavity, as cross-sections show at once. As the development progresses, the embryo lengthens out, but the swelling caused by the yolk persists for a long period, the yolk material being only gradually resorbed by the embryo; the swelling is readily recognized, even up to larval stages.

The secondary type of vertebrate embryo is found in elasmobranchs and amniota. In elasmobranch,% when the embryo appears it occupies only a small part of the ovum, which is very large and contains much yolk. Soon alter the ai)pe{irance of the medullary gr(X)ve, the head of the embryo l)cgins to gr<jw forward entirely free from and alxjve the yolk; and by the time the medullary groove is converted into the medullary canal the tail begins to grow backward in a similar manner independently of the yolk; hence, only the central region of the embryo remains connected with the yolk. As the growth of the embryo continues, while the area of its Ixxly attached to the yolk increases very little in size, it follows that the connection l)ecomes relatively smaller, until it becomes merely a narrow stalk jis com|>fired either with the embr^'o or the mass of yolk. The traditional and often-reix^ated description of the separation of the embryo from the yolk attributes the separatiort to a folding oflF of tlit^ emlirvo by the germ-layers being tucked in under the embryonic anlage, but it seems to me that the process is only apparent, and that it is l)y its own growtli, as al)ove describe<l, that the ombryo l)ecomes partly separated from the yolk; and I hold the same view as regards the amniota.

The yolk is covered bv the extra-embrvonic extensions of the mesoderm and o(*toderm, the yolk proi>er l)eing, of course, entodenn. If the mesoderm develops a coelomatic fissure around the yolk, we have the non-embryonic parts of the ovum converted into a double sac; an outer sac formed bv the united ectodenn and mescnlerm (somatopleure), and an inner sac of mesotlerm filleil with the yolk mass (vitelline entoderm), the two representing the splanchnopleure. The outer sac in all vertebrates may be called the clwrion^ the name by which it is known in mammalia; the inner sac is the yolk-sac or tuubilical vesicle.

In umniota^ the separation of the embryo from the yolk take^ place in the same general maimer as just described for elasmobranchs, but there are additional complications due to the development of the amnion and allantois taking place vevy early — see the following division of this chapter.

Form of the Axnniote Embryo

It is not proposed to give here a comparative account of the forms of amniote embryos at successive stages, but merely to briefly indicate the characteristics of the stage in which all the principid anhiges of the primary organs are present, but not si)ecialized. The stage may be taken to be that of the hen's ovum at fifty to sixty hours of incubation, Fig. 150. The bhistoderm reaches at this time over nearly half of the yolk, the extreme margin of the opaque area being near the equator, but the vascular area is much smaller, being only about 20 mm. in diameter ; still smaller is the pear-shaped area pellucida, in the centre of which lies the rapidly growing embryo. At this period the vascular area may be said to be in the stage of its most complete development ; for though it will afterward become larger, it will at the same time become less definite and relatively less important. The arterial system already has its main trunks. Fig. 157.-4. r., and the main stems of the omphalo-mesaraic veins, om. ]', are differentiated. As regards the embryo the most striking features are the advanced development of the head and the slight differentiation of the tail. The head has grown forward so as to be entirely free from the yolk, and is turned so that its left side rests upon the yolk, and as the tail end of the embryo still rests symmetrically upon the yolk, it follows that the intermcliate portion of the body is twistecl. This warping or torsion of the embryo, in order that the side of the flattened head may rest upon the yolk, occui^s in Sauropsida and to a slight extent in placental mammals, but not among any of the Ichthyopsida. We must, therefore, regard it as a special feature of the amniote embryo, which has been lost in the placental mammals, probably as a result of the loss of food yolk in the ovum. The head is remarkable for the advanced differentiation of its parts; the anlages of the eye. Fig. 150, i, and ear, O^ are present; four branchial pouches are developed, 6r^; the heart is large and already bent, Ht; the medullar^" tube is ver>" much dilated and distinctly divided into its three primary' vesicles, if, Mb, Hh, The head is also bent at the region of the mid-brain, J/ 6, so as to form almost a right angle with the axis of the hind-brain, 7/6, and neck. This head-bend or cervical flexure is highly characteristic of all vertebrates; it is beautifully shown in elasmobranch embryos, and can be easily recognized in all classes. It is a bend in the median plane of the embryo by which the end of the head is brought over toward the heart, Ht. Following along backward we encounter the first distinct segments just behind the otocyst, O/, and can follow them some distance l>ehind the vitelline arteries, until they merge into the imdivided segmental zone, Ar; the limit of the bodv of the embrvo is already indicated bv the parietal zone, but the zone will bo encroached uix)n by the vascular area, and the whole zone of this stage is not destined to be included in the body of the embryo.

In a sheep embryo,* although the fundamental characteristics are the same, there are many minor differences both from the chicken and the rabbit. The most striking peculiarities of the embryo are due to the foetal appendages, the development of which presents special modifications in ruminants, as more fully described in the next division of this chapter; the yolk-sac is long and narrow, and is connected by a broad twisted yolk-stalk with the embryo; the allantois has already become a very large transversely expanded vesicle; the amnion invests the embryo closely and gives off a long cord (Amnionstrang) ^ by which it is still attached to the chorion. The embr}o, 5 mm. in length, is curving throughout its length; the head-bend is developed, and consequently the end of the head lies near the heart ; the torsion of the whole embryo is very marked, the dorsal side of the fore-brain fiicing us, of the neck being turned away from us, of the tail facing us again ; the embrj'^o makes nearly one complete spiral turn. The head is small, laterally compressed, and less advanced than in the chick described above, for the anlage of the eye is only just begun ; that of the ear is not differentiated, and the first two visceral arches are present, while the third is only just l)eginning. The medullary groove is still open in the region of the forebrain, and widely open at its tail end, but closed throughout the rest of its length ; there are fourteen segments ; none of the vessels yet contain any red blood.

Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding. (More? Embryology History | Historic Embryology Papers)